Rotor Earth Fault Protection of Generator:

As we know already that field circuits are operated unearthed and it is not necessary to trip such a circuit completely if there is only one earth fault but the relay should be provided to give an indication that the fault has occurred so that the generator may be taken out of service at leisure since the incidence of second fault would cause serious damage. There are two methods of rotor earth fault protection of Generator.

First Method:

One method of detecting Rotor earth fault protection on rotor circuit is illustrated in Fig. 11.18 (a). In this arrangement a high resistance is connected across the rotor circuit and its midpoint is grounded through a sensitive relay. The relay detects the earth fault for most of the rotor circuit except the rotor centre point.

Second Method:

Other methods of rotor earth fault protection include dc injection method and ac injection method. The arrangement is illustrated in Fig. 11.18 (b). In this arrangement either dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor (in case of ac voltage). A single earth fault in the rotor circuit will complete the circuit comprising the voltage source (ac or dc), sensitive overvoltage relay and earth fault and thus earth fault will be sensed by the relay.

DC injection method is simple and has no problems of leakage currents. If direct current is used, the overvoltage relay can be more sensitive than if alternating current is used, with ac, the relay must not pick up on the current that flows normally through the capacitance to ground, and care must be taken to avoid resonance between the capacitance and the relay inductance.